Royals physio Jon Fearn told the club's official website, www.readingfc.co.uk: "Glen and I recently saw a specialist in Achilles management problems.
"He thinks that what we may have is a small tear in the tendon that has not responded to the treatment we have given him."
He continued: "That can happen so the next stage is is to go into the tendon and cut out the damaged tissue.
"Then we have to wait for the tendon to heal. Tendon takes twice as long as muscle, so we are looking at probably four to six months' recovery time.
"That will still leave some time toward the latter part of the season.
"I think a lot of talk I have seen about Glen has been very pessimistic but it has been a very, very nasty problem.
"You would never take the decision in the summer to go down this route because there was every chance it would heal without it.
"That's not been the case but some people don't respond to certain treatment and now we'll go down this line."
Also needing surgey is new signing Kalifa Cisse, he needs an operation on cartilage in his knee and will be out for a month.